Synthesis of cubic diamond in the graphite-magnesium carbonate and graphite-K 2 Mg(CO 3 ) 2 systems at high pressure of
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Synthesis of cubic diamond in the graphite-magnesium carbonate and graphite-K2 Mg(CO3 )2 systems at high pressure of 9–10 GPa region T. Taniguchi National Institute for Research in Inorganic Materials, 1-1 Namiki Tsukuba Ibaraki 305, Japan
D. Dobson, A. P. Jones, R. Rabe, and H. J. Milledge Department of Geophysical and Geological Science, University College London, Gower Street, London WC1E 6BT, United Kingdom (Received 9 August 1995; accepted 6 April 1996)
Cubic diamond was synthesized with two systems, (1) graphite with pure magnesium carbonate (magnesite) and (2) graphite with mixed potassium and magnesium carbonate at pressures and temperatures above 9.5 GPa, 1600 ±C and 9 GPa, 1650 ±C, respectively. At these conditions (1) the pure magnesite is solid, whereas (2) the mixed carbonate exists as a melt. In this pressure range, graphite seems to be partially transformed into hexagonal diamond. Measured carbon isotope d13 C values for all the materials suggest that the origin of the carbon source to form cubic diamond was the initial graphite powder, and not the carbonates.
I. INTRODUCTION
Recently, it has been reported that the conversion reaction from graphite to diamond takes place at high pressure (HP) (8 GPa) and high temperature (HT) (2100 ±C) in the presence of some inorganic compounds as solvent catalyst.1–3 Following the first discovery, several experimental investigations for the growth and nucleation of synthetic diamond with the inorganic compounds have been reported.4–6 Among the several inorganic compounds identified in these pilot studies, the graphite-carbonate system is particularly interesting, because carbonate minerals are common in the earth. The reported studies for the carbonate-graphite system have been carried out in the pressure range up to 8 GPa at HT near 2150 ±C.1,3 As mentioned in these papers, the synthesis temperature for diamond in this system is much higher than that for the formation of natural diamond. However, it was found in the other inorganic compound catalyst system that the lower boundary of the synthesis temperature for diamond becomes lower as the pressure is increased.3 In order to clarify the reaction boundary for diamond formation in the presence of carbonate, it is important to carry out synthesis experiments under a wider range of pressures. On the other hand, the lower limit of temperature required to form diamond has been considered to be related to the melting temperature of the catalytic solvent materials.7 It is known that the melting temperature of binary carbonate systems becomes lower by involving an alkali component due to their eutectic or peritectic relationship.8 Therefore, the study of the formation of diamond in the mixed carbonate system with a eutectic 2622
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J. Mater. Res., Vol. 11, No. 10, Oct 1996
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relationship is interesting for determining the lower boundary of reaction temperature. With the above perspective, we carried out an investigation t
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